Tuning of fe catalysts for growth of spin-capable carbon nanotubes

ABSTRACT

Growing spin-capable multi-walled carbon nanotube (MWCNT) forests in a repeatable fashion will become possible through understanding the critical factors affecting the forest growth. Here we show that the spinning capability depends on the alignment of adjacent MWCNTs in the forest which in turn results from the synergistic combination of a high areal density of MWCNTs and short distance between the MWCNTs. This can be realized by starting with both the proper Fe nanoparticle size and density which strongly depend on the sheet resistance of the catalyst film. Simple measurement of the sheet resistance can allow one to reliably predict the growth of spin-capable forests. The properties of pulled MWCNTs sheets reflect that there is a relationship between their electrical resistance and optical transmittance. Overlaying either 3, 5, or 10 sheets pulled out from a single forest produces much more repeatable characteristics.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This Application for Patent claims the benefit of priority from U.S.Provisional Patent Application Ser. No. 61/250,827, filed Oct. 12, 2009,entitled “Tuning Of Fe Catalysts For Growth Of Spin-Capable CarbonNanotubes,” which provisional patent application is commonly assigned tothe assignee of the present invention, and which disclosure isconsidered part of and is incorporated by reference in its entirety inthe disclosure of this application.

BACKGROUND

1. Field of Invention

The present invention is a new process for growing spin-capablemulti-walled nanotube (MWCNT) forests in a repeatable fashion.

2. Background of the Invention

Since the discovery that one can directly pull multi-walled carbonnanotubes (MWCNTs) into the forms of sheets and/or yarns from forests[Jiang K, Li Q, Fan S. Spinning continuous carbon nanotube yarns. Nature2002; 419:801 (“Jiang 2002”)], the majority of research effort has beenfocused on the physical, mechanical and electrical properties of theseMWCNT yarns and sheets as well as their various applications [Zhang M,Atkinson K R, Baughman R H. Multifunctional carbon nanotube yarns bydownsizing an ancient technology. Science 2004 306:1358-1361 (“M Zhang2004”); Zhang X, Jiang K, Feng C, Liu P, Zhang L, Kong J, et al.Spinning and processing continuous yarns from 4-Inch wafer scalesuper-aligned carbon nanotube arrays. Adv Mater 2006 18:1505-1510 (“XZhang 2006”); Zhang X, Li Q, Tu L, Li Y, Coulter J Y, Zheng L, et al.Strong carbon-nanotube fibers spun from long carbon-nanotube arrays.Small 2007 3:244-248 (“X Zhang 2007”); Li Q, Li Y, Zhang X,Chikkannanavar S B, Zhao Y, Dangelewicz A M, et al. Structure-dependentelectrical properties of carbon nanotube fibers. Adv Mater 200719:3358-3363 (“Li 2007”); Liu K, Sun Y, Chen L, Feng C, Feng X, Jiang K,et al. Controlled growth of super-aligned carbon nanotube arrays forspinning continuous unidirectional sheets with tunable physicalproperties. Nano Lett 2008 8:700-705 (“Liu 2008”)]. In particular, manyapplications have been demonstrated that are only possible because ofthe unique properties of CNTs [Baughman R H, Zakhidov A A, de Heer W A.Carbon nanotubes—the route toward applications. Science 2002 297:787-792(“Baughman 2002”)]. These potential applications are widespread andinclude transparent electrodes, flexible displays and compositematerials [Baughman 2002; Zhang M, Fang S, Zakhidov A A, Lee S B, AlievA E, Willaims C D, et al. Strong, transparent, multifunctional, carbonnanotube sheets. Science 2005 309:1215-1219 (“M Zhang 2005”); UlbrichtR. Polymeric solar cells with oriented and strong transparent carbonnanotube anode. Phys Stat Sol B 2006 243:3528-3532; Gruner G. Carbonnanotube films for transparent and plastic electronics. J Mater Chem2006 16:3533-3539; Kaempgen M, Duesberg G S, Roth S. Transparent carbonnanotube coatings. Appl Surf Sci 2005 252:425-429; Cheng Q, Wang J,Jiang K, Li Q, Fan S. Fabrication and properties of aligned multi-walledcarbon nanotube-reinforced epoxy composites. J Mater Res 200823:2975-2983].

Applicant believes much less research effort has centered on thecritical growth factors that determine whether or not the MWCNTs in aforest can be pulled or spun. These factors remain somewhat obscure todate because only a few research groups [Jiang 2002; M Zhang 2004; XZhang 2006; X Zhang 2007] have actively investigated the protocols togrow “spin-capable” MWCNTs over the past half decade. While those groupshave used different growth conditions (catalysts, gases andtemperatures) the data and understanding still need improvement. Forexample: while it is well known that MWCNTs will grow on Fe or Al₂O₃/Feusing ethylene or acetylene as a carbon source and argon, helium, and/orhydrogen as the carrier gas, the most important factors for growingspin-capable forests are not well known. These still need to beinvestigated further. Some reports [Jiang 2002; X Zhang 2006; Li 2007;Liu 2008, Li Q, Zhang X, DePaula R F, Zheng L, Zhao Y, Stan L, et al.Sustained growth of ultralong carbon nanotube arrays for fiber spinningAdv Mater 2006 18:3160-3163] suggest that the spinning capabilities ofMWCNT forests result when the CNTs in the forest become “super-aligned”arrays. The idea is that the CNTs must be very well aligned parallel toone another and in addition have a high density in the forest.

Further study also indicates that super-aligned CNTs are generallyformed when there is a high CNT nucleation density (or high density ofcatalyst nanoparticle sites) [Liu 2008; Nessim G D, Hart A J, Kim J S,Acquaviva D, Oh J, Morgan C D, et al. Tuning of vertically-alignedcarbon nanotube diameter and areal density through catalystpre-treatment. Nano Lett 2008 8:3587-3593, 15; Futaba D N, Hata K, NamaiT, Yamada T, Mizuno K, Hayamizu Y, et al. 84% catalyst activity ofwater-assisted growth of single walled carbon nanotube forestcharacterization by a statistical and macroscopic approach. J Phys ChemB 2006 110:8035-8038]. As a consequence, there have been somesignificant efforts to increase the nucleation density by controllingboth the size and size distribution of Fe catalyst nanoparticles throughvarious methods. These methods have included gas-assisted pretreatmentmethods [Cantoro M, Hofmann S, Pisana S, Ducati C, Parvez A, Ferrari AC, et al. Effects of pre-treatment and plasma enhancement on chemicalvapor deposition of carbon nanotubes from ultra-thin catalyst films.Diamond Relat Mater 2006 15:1029-1035; Zhang G, Mann D, Zhang L, JaveyA, Li Y, Yenilmez E, et al. Ultra-high-yield growth of verticalsingle-walled carbon nanotubes: hidden roles of hydrogen and oxygen.Proc Natl Acad Sci USA 2005 102:16141-16145; Pisana S, Cantoro M, ParvezA, Hofmann S, Ferrari A C, Robertson J. The role of precursor gases onthe surface restructuring of catalyst films during carbon nanotubegrowth. Physica E 2007 37:1-5] the use of various catalysts [Wang Y, LuoZ, Li B, Ho P S, Yao Z, Shi L, et al. Comparison study of catalystnanoparticle formation and carbon nanotube growth: Support effect. JAppl Phys 2007 101:124310 (“Wang 2007”); Hofmann S, Cantoro M,Kleinsorge B, Casiraghi C, Parvez A, Robertson J, et al. Effects ofcatalyst film thickness on plasma-enhanced carbon nanotube growth. JAppl Phys 2005 98:034308], and the optimization of the catalyst filmthickness [Chakrabarti S, Kume H, Pan L, Nagasaka T, Nakayama Y. Numberof walls controlled synthesis of millimeter-long vertically alignedbrushlike carbon nanotubes. J Phys Chem C 2007 111:1929-1934; Patole SP, Alegaonkar P S, Shin H C, Yoo J B. Alignment and wall control ofultra long carbon nanotubes in water assisted chemical vapourdeposition. J Phys D: Appl Phys 2008 41:155311].

It is therefore important to study how the nucleation density isaffected by controlling the size and size distribution of Fe catalystparticles and optimizing gas flow rates used in the growth of CNTs. Thisinformation is valuable in determining optimal conditions for developingsuper-aligned arrays of CNTs as well as improving the ability to pullCNTs out from CNT forests. Understanding the mechanism by whichsuper-aligned arrays and high nucleation density can be achieved iscritical for controlling and stabilizing the growth process for thespinning CNTs as well as providing a bigger opportunity ranging fromacademic research to commercial applications of the CNT sheets andyarns.

SUMMARY OF THE INVENTION

The present invention is a new process for growing spin-capablemulti-walled nanotube forests in a repeatable fashion.

In the present invention, Applicant has identified roles that the Fecatalyst film and carrier gas play in setting the alignment, thenucleation density, the CNT diameter, and the “spinning capability” ofMWCNT forests. Applicant has also shown that a sheet resistancemeasurement is a method of measuring the iron catalyst film thicknessand can be used to predict whether the catalyst film will be suitablefor growing a forest that is spin-capable.

It has been found that growing spin-capable multi-walled carbon nanotube(MWCNT) forests in a repeatable fashion can be realized by starting withboth the proper Fe nano-particle size and density which strongly dependon the sheet resistance of the catalyst film, annealing time, and gasconditions. A measurement of the sheet resistance of Fe catalyst filmcan allow one to reliably predict the growth of spin-capable forests.

In general, in one aspect, the invention features a method to measurethe suitability of a catalyst film for growing spinable carbon nanotube(CNT) forest that includes:

An Fe catalyst film having a sheet resistance in the range of about 10kΩ/square [˜3 nm] to about 1 MΩ/square [˜9 nm] is deposited on athermal-oxidized silicon substrate, and the conductivity, sheetresistance, resistivity of said film is measured. Then thecatalyst-incorporated substrate is put on a heater block in a chemicalvapor deposition (CVD) chamber, the heater block is heated up to atemperature (780° C.) for the growth of CNT forest under a gas mixtureof hydrogen (100 sccm) and helium (20 slm) as a carrier, and thesubstrate is held in the CVD chamber at the temperature for the growthof CNT forest during 0˜30 minutes under the carrier gases. A gas mixtureconsisting of acetylene (700 sccm) as a CNT-forming precursor and thecarrier gases is supplied during 1˜30 minutes for the growth of thespin-capable CNT forest on the substrate such that the CNT forest arevery well aligned in a direction perpendicular to said substrate. Abundle of CNTs is pulled out from the CNT forest, forming CNT sheets oryarns comprised of the CNT bundles which are entangled end to end, inwhich the CNTs are mostly oriented parallel to each other along thedrawing direction.

The physical, mechanical and electrical properties of MWCNT yarns andsheets as well as their various applications including electrodes,flexible displays and composite materials have been studied because ofthe unique properties of CNTs including long ballistic conduction.As-formed sheets or yarns pulled out from the forest can be applied tothe electrodes for solar cells, LEDs, fuel cells, and lithium batteriesby incorporating the sheets or yarns into the device.

In general, in another aspect, the invention features a method forforming a hole conducting layer with CNTs in a photovoltaic cellincluding:

An electron-collecting electrode having aluminum deposited on asubstrate, photoactive layer, which is made of a conjugated polymer,preferably a polythiophene derivative, as an electron donor and afunctionalized fullerene derivatives, including PCBM as an electronacceptor, is coated onto the electron-collecting electrode, a holeconducting layer, which is laid a hole conducting material such asPEDOT, ITO and p-type transparent metal oxides over CNT sheets (oryarns) and on the contrary, is formed on the photoactive layer, andfinally a hole-collecting electrode is formed on the hole conductinglayer.

Compared with conventional methods in fabrication of solar cell, thepresent invention is contrary to the conventional methods in themanufacturing sequence. This leads to an easier preparation oflarge-area flexible solar cells because the CNT sheets can becontinuously pulled from the forest and put on a sub-layer. Furthermore,the sheets or yarns can be electro-deposited with metal oxide, formingCNT sheets or yarns encapsulated by metal oxides comprised of coaxialmetal oxide/CNT sheets or yarns. The coaxial sheets or yarns can beincorporated into the hole conducting layer or photoactive layer insolar cells, and also employed as an electrode of fuel cells and lithiumbatteries.

In general, in one aspect, the invention features a method that includesdepositing a catalyst film on a substrate to form acatalyst-incorporated substrate. The catalyst film has a sheetresistance in the range of about 10 kΩ/square to about 1 MΩ/square. Themethod further includes measuring the conductivity, sheet resistance,and resistivity of the catalyst film. The method further includesgrowing a CNT forest on the catalyst-incorporated substrate. The methodfurther includes pulling out a bundle of CNTs from said CNT forest. Themethod further includes forming a CNT sheet or yarn.

Implementations of the invention can include one or more of thefollowing features:

The step of measuring can include using a four-point probe.

The step of measuring can include using an eddy current probe.

The catalyst film can include iron. The sheet resistance of the catalystfilm can be in the range of about 10 kΩ/square to about 1 MΩ/square.

The substrate can include an oxidized material of silicon, a metal, ametal oxide, glass, or a combination thereof.

The catalyst film can include an iron deposited on the substrate by anelectron-beam evaporation process, a sputtering process, or a chemicalvapor deposition process.

The bundle of CNTs can include multi-walled CNTs.

The step of growing the CNT forest can include putting thecatalyst-incorporated substrate on a heater block in a chemical vapordeposition (CVD) chamber. The step of growing the CNT forest can furtherinclude heating the CVD chamber up to a temperature for the growth ofCNT forest. The step of growing the CNT forest can further includeholding the substrate in the CVD chamber at the temperature for thegrowth of CNT forest. The step of growing the CNT forest can furtherinclude providing a gas mixture consisting of CNT-forming precursors andcarrier gases. The step of growing the CNT forest can further includegrowing the CNT forest on the substrate such that the CNT forest aresubstantially aligned in a direction perpendicular to the substrate.

The hold temperature of the heater block in the CVD chamber can bebetween room temperature and the temperature for the growth of CNTforest.

The temperature for the growth of the CNT forest can be between about650° C. and about 850° C.

The heating rate up to the temperature for the growth of CNT forest canbe between about 10° C./minute and about 1000° C./minute.

The substrate that is heated up to the temperature for the growth of CNTforest can stay less than 30 minutes in carrier gases.

Before growing the CNT forest, carrier gases can be supplied into theCVD chamber.

The CNT-forming precursor can include acetylene. The carrier gases caninclude hydrogen and an inert gas.

The inert gas can be helium, nitrogen, or argon.

The flow rate of the acetylene can be between about 200 sccm and about1000 sccm. The flow rate of the hydrogen can be between about 50 sccmand about 1000 sccm. The flow rate of the inert gas can be between about1 slm and 30 slm.

The CNTs of the bundle of CNTs can include multi-walled CNTs that areentangled end to end. The multi-walled CNTs can be substantiallyparallel to each other along the drawing direction.

In general, in another aspect, the invention features a method offorming a hole conducting layer in a photovoltaic cell that includesforming a first electrode. The method further includes forming anelectron donor/acceptor composite layer. The method further includesforming a hole conducting layer stacked with a hole conducting materialover CNT sheets or yarns. The method further includes forming a secondelectrode on the contrary.

Implementations of the invention can include one or more of thefollowing features:

The hole conducting material can bepoly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), indium tinoxide (ITO) or a p-type transparent metal oxide.

The p-type transparent metal oxide can have a work function of over 4.5eV.

The p-type transparent metal oxide can be iron oxide, nickel oxide, tinoxide, vanadium oxide, zinc oxide, or a combination.

The CNT bundles in the CNT sheet or yarn can be substantially aligned inthe same direction or cross each other perpendicularly.

The hole conducting layer can be a triple-layer in which the CNT sheetsor yarns is laid between the hole conducting materials.

In general, in another aspect, the invention features a method ofincorporating a hole conducting material into an electron donor/acceptorcomposite layer in a photovoltaic cell. The method includes forming afirst donor/acceptor composite. The method further includes forming ahole conducting layer that includes CNT sheets. The CNTs areencapsulated by metal oxides that include metal oxide/CNT sheetcomposites. The method further includes forming a second donor/acceptorcomposite.

Implementations of the invention can include one or more of thefollowing features:

The metal oxide can be a p-type transparent metal oxide with workfunction of over 4.5 eV.

The p-type transparent metal oxide can be iron oxide, nickel oxide, tinoxide, vanadium oxide or zinc oxide.

The metal oxide can be a transition metal oxide.

The metal oxide can be coated onto the CNTs by a sputtering process, anelectro-deposition process, a chemical vapor deposition (CVD) process,or a combination thereof.

The CNTs can be encapsulated by metal oxides by a process that includescoating a metal onto the CNTs. The process that includes coating a metalonto the CNTs can be a sputtering process, an electro-depositionprocess, a CVD process, or a combination thereof. The step of oxidizingthe metal can use oxidants.

The oxidants can be oxygen, ozone, hydrogen peroxide, or nitric acid.

In general, in another aspect, the invention features a method ofmanufacturing an anode for fuel cells and lithium batteries. The methodincludes pulling out a bundle of CNTs from said CNT forest. The methodfurther includes forming CNT sheets. The method further includes formingmetal oxide/CNT sheet composites that include CNT sheets. The CNTs areencapsulated by metal oxides.

Implementations of the invention can include one or more of thefollowing features:

The metal oxide can include a transition metal oxide.

The transition metal oxide can be manganese oxide, nickel oxide, cobaltoxide or iron oxide.

The metal oxide can be coated onto the CNTs by a sputtering process, anelectro-deposition process, a chemical vapor deposition (CVD) process,or a combination thereof.

The CNTs can be encapsulated by metal oxides by a process that includescoating a metal onto the CNTs. The process that includes coating a metalonto the CNT can be a sputtering process, an electro-deposition process,a CVD process, or a combination thereof. The step of oxidizing the metalcan use oxidants.

The oxidants can be oxygen, ozone, hydrogen peroxide, or nitric acid.

The foregoing has outlined rather broadly the features and technicaladvantages of the invention in order that the detailed description ofthe invention that follows may be better understood. Additional featuresand advantages of the invention will be described hereinafter that formthe subject of the claims of the invention. It should be appreciated bythose skilled in the art that the conception and the specificembodiments disclosed may be readily utilized as a basis for modifyingor designing other structures for carrying out the same purposes of theinvention. It should also be realized by those skilled in the art thatsuch equivalent constructions do not depart from the spirit and scope ofthe invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of theinvention, reference will now be made to the accompanying drawings inwhich:

FIGS. 1A-1B show the sheet resistance of Fe films. FIG. 1A shows R_(s)measurements of various Fe films. The thickness listed is obtained fromthe quartz-crystal sensor during deposition. FIG. 1B shows a wafer mapof the sheet resistance on the 5 nm thick Fe film [kΩ/sq]. Each chip isapproximately 1×0.7 cm².

FIGS. 2A-2C show the relationship between the spinning length of CNTsand R_(s) of Fe catalyst films. FIG. 2A is a histogram showing thelength of pulled CNT sheets vs. R_(s) of the Fe catalyst films. Each barrepresents the spinning length when a CNT sheet is pulled out of theforest. (The bar 201 denotes the spinning length when the sheet waspulled at the first trial, bar 202 denotes the second, bar 203 denotesthe third, and bar 204 denotes the fourth.) The x-axis scale has boththe R_(s) (upper) and quartz crystal monitor thickness (lower). FIG. 2Bis a picture showing a 1 m long sheet 205.

FIG. 2C is a plot of sheet resistance versus Fe film thickness by AFMfor as-deposited films.

FIGS. 3A-3G are SEM images of CNTs grown on chips with various Fe films.FIG. 3H is a plot showing the height rate (curve 301) and growth rate(curve 302) of CNT forests grown during 5 minutes under the optimizedcondition.

FIGS. 4A-4G are AFM (*-1) and SEM (*-2) images showing the surfacemorphology of annealed Fe films with sheet resistance. The scale bar isthe same in AFM and SEM images, respectively. Histograms of the Fenanoparticle size distributions as a function of R_(s) are also shown inFIGS. 4A-4G.

FIGS. 5A-5E are SEM images of CNTs grown on chips with various Fe filmsunder He gas. FIG. 5F is a plot showing the height (μm) of CNT forestsgrown on chips during 5 minutes under H₂/He (curve 501) and He gas(curve 502), respectively. This corresponds to the height of CNT forestsgrown under H₂/He gas in FIG. 3H.

FIGS. 6A-6C are SEM images showing the surface morphology of annealed Fefilms under He gas.

FIGS. 7A-7B show the optical transmittance at 550 nm of MWCNT sheets asa function of the electrical sheet resistance of the MWCNT sheet. FIG.7A is a plot for single layer MWCNT sheets from forests grown on variousFe films. FIG. 7B is a plot for multi-sheets from the Fe catalyst filmof 162 kΩ/sq.

FIGS. 8A-8D are SEM images showing the morphology of MWCNT multi-sheets.These SEM images were obtained from CNT sheets on respective electrodes.

FIGS. 9A-9G are TEM images and histograms of the average diameters (andthe standard deviation) of CNTs associated with various R_(s) withGaussian fitting (solid lines).

FIG. 10A is MWCNT sheets 1001 transferred onto glass substrates (withcopper tape 1002) for a 4-point probe measurement. FIG. 10B is an I-Vcurve measured by 4-point probe.

FIG. 11A are TEM images of multi-walled carbon nanotubes grown on aniron film. FIG. 11B is a histogram of the average diameter and thestandard deviation of the MWCNTs in this film. FIG. 11C is a plot ofRaman intensity versus Raman shift for this film.

FIG. 12A shows a sheet 1201 pulled from a multi-walled carbon nanotubeforest 1202 (such as shown in FIG. 11A. FIG. 12B shows a yarn 1203pulled from a multi-walled carbon nanotube forest.

FIG. 13 illustrates the believed mechanism in which MWCNTs are pulledout from the multi-walled carbon nanotube forests.

FIGS. 14A-14J are TEM images that model the pulling the sheet from amulti-walled carbon nanotube forest.

FIGS. 15A-15B are TEM images that model the pulling the sheet from amulti-walled carbon nanotube forest.

FIGS. 16A-16D are MWCNT sheets (1, 3, 5, and 10, respectively)transferred onto glass substrates with the corresponding I-V curvemeasured by 4-point probe. FIG. 16E shows the optical transmittance as afunction of wavelength of these MWCNT sheet of FIGS. 16A-16D. FIG. 16Fshows the optical transmittance at 550 nm of MWCNT sheets as a functionof the electrical sheet resistance of these MWCNT sheet of FIGS.16A-16D.

FIG. 17 shows a flexible electrode that can utilize the MWCNT yarns andsheets.

FIG. 18 shows a nanowire that can utilize the MWCNT sheets and yarns.

FIG. 19 illustrates a photovoltaic cell in which a MWCNT sheet or yarncan be utilized as a hole conducting layer.

FIG. 20 illustrates an alternative photovoltaic cell in which a MWCNTsheet or yarn that can be utilized as a hole conducting layer.

FIGS. 21A-21C are illustrations of MWCNT sheet that can be utilized as ahole conducting layer in a photovoltaic cell. FIG. 21A is a top view ofa one-dimensional structure. FIG. 21B is a top view of a two-dimensionalstructure.

FIG. 21C is a side view of the two-dimensional structure of FIG. 21B.

FIG. 22 illustrates a solar cell in which a MWCNT sheet or yarn can beutilized (in the form of a metal oxide/CNT sheet composite).

FIG. 23 illustrates an alternative embodiment of a solar cell in which aMWCNT sheet or yarn can be utilized (in the form of a metal oxide/CNTsheet composite).

FIG. 24 illustrates an alternative embodiment of a solar cell in which aMWCNT sheet or yarn can be utilized (in the form of a metal oxide/CNTsheet composite).

FIG. 25 illustrates a metal oxide/CNT sheet composite.

FIGS. 26A-26C are TEM images of embodiments the metal oxide/CNT sheetcomposite illustrated in FIG. 25, namely a manganese oxide/CNT sheetcomposite, a cobalt oxide/CNT sheet composite, and a nickel metal/CNTsheet composite, respectively.

DETAILED DESCRIPTION

The present invention is a new process for growing spin-capablemulti-walled nanotube forests in a repeatable fashion.

While the making and/or using of various embodiments of the presentinvention are discussed below, it should be appreciated that the presentinvention provides many applicable inventive concepts that may beembodied in a variety of specific contexts. The specific embodimentsdiscussed herein are merely illustrative of specific ways to make and/oruse the invention and are not intended to delimit the scope of theinvention.

Materials and Preparation of Samples

CNTs were grown from iron films, which were deposited on Si substrateswith an oxidized layer of thickness 400 nm by electron-beam evaporation.The thickness of the Fe thin film was varied in the range of 3˜7 nm andwas monitored by a quartz crystal sensor fixed inside the e-beamevaporation chamber. MWCNT growth was performed in a quartz andstainless steel cylindrical CVD chamber at atmospheric pressure usingflows of C₂H₂ (acetylene) and He gas either with or without H₂ gas. Theiron films were introduced into the CVD chamber and ramped to the setpoint temperature of 780° C. at a ramping rate of 50° C./min whileflowing He (20 slm) and H₂ (100 sccm). The growth of CNTs was carriedout at the same temperature and pressure by adding acetylene gas (700sccm) to the flow for 5 min. The acetylene gas flow was turned off andthe sample was cooled to below 100° C. with the He gas flow continuing.The CNT sheets were spun from CNT forests with a rod rotating once anddrawing at ˜1 m/min.

When investigating the surface morphology and size of iron particles,the iron films were introduced into the same CVD chamber and ramped tothe set point temperature of 780° C. while flowing He (20 slm) with orwithout H₂ (100 sccm), and then allowed to cool below 100° C.

FIG. 11A are TEM images of multi-walled carbon nanotubes grown on aniron film. The growth height is 420 μm during the 5 minute growthperiod. The growth rate was 1.4 μm/sec. The MWCNT area density was1.1×10¹¹ tubes per square centimeter.

FIG. 11B is a histogram of the average diameter (13.0 nm) and thestandard deviation (1.3 nm) of the MWCNTs in this film. FIG. 11C is aplot of Raman intensity versus Raman shift for this film(I_(G)/I_(D)=1.76).

Measurements

Scanning electron microscopy (SEM) and transmission electron microscopy(TEM) images were taken using a Zeiss Supra-40 and a FEI Tecnai-G2instruments, respectively. The TEM specimens were prepared by dispersingthe CNTs in methanol and placing a few drops onto TEM copper microgrids. Raman spectroscopy was performed using a Nicolet Almega XRdispersive Raman spectrometer with 532-nm excitation wavelength. Atomicforce microscopy was carried out using a Veeco Dimension 3100. The sheetresistance of as-deposited iron films was measured using an ALESSI4-point probe. The areal density of CNTs was calculated by directlycounting the holes resulting from pulling out CNTs from the substrate.

Electrical measurements of the CNT sheets pulled from forests wereconducted using the 4-point probe method at room temperature. The MWCNTsheets pulled from ˜400 um tall CNT forests were transferred onto glasssubstrates. Four copper tape electrodes were attached for a 4-pointprobe measurement. The sheets tested here were between 4 and 10 mm inwidth and between 8 and 14 mm in length. The sheet resistance of CNTsheets is defined as RW/L (R=measured resistance, W=sheet width, andL=sheet length).

The Growth of Spin-Capable MWCNTs

To understand the impact of the Fe film thickness on the size and sizedistribution of Fe nanoparticles which are ultimately connected to themorphology, diameter and forest height of CNTs, the thickness of the Fefilm must be measured. The Fe film thickness measured using aQuartz-crystal sensor fixed inside the e-beam evaporation chamber is notgenerally consistent enough due to both poor within-wafer uniformity andrelatively large wafer-to-wafer variations. FIG. 1 demonstrates this.FIG. 1A shows how the Fe film's sheet resistance (R_(s)) is not uniformacross the wafer with each curve representing the length of the pulledsheet (curves 101, 102, 103, 104, 105, and 106 are 3 nm, 3 nm, 4 nm, 5nm, 6 nm, and 7 nm, respectively). Since the Fe film is deposited on aSiO₂ insulator, the sheet resistance will directly depend upon thethickness and continuity (conductivity) of the Fe film. The differencein R_(s) in different regions of the wafer clearly indicates importantchanges in the Fe film across the wafer.

The difference is primarily related to the Fe film thickness. There canalso be a substantial difference between wafers having the same nominalfilm thickness (as indicated by the quartz-crystal sensor). For example,two wafers with nominally 3 nm Fe films (by the quartz-crystal sensor)have widely disparate R_(s) (˜1 MΩ/sq and ˜9 MΩ/sq, as shown in curves102 and 101, respectively).

To address this, the wafers with as-deposited Fe film were sectionedinto small chips (˜1.0×0.7 cm²) as shown in FIG. 1B. The R_(s) of eachchip was approximately constant and could easily be measured using afour-point probe. Then the film thickness was measured by atomic forcemicroscopy (AFM) so that the R_(s) of each chip could be correlated withthe AFM thickness.

The plot is shown in FIG. 2C and the correlation found there is:

R _(s)(kΩ/sq)=3.0×10⁵× Thickness (nm)^((−4.7))  (1)

Sheet resistance measurements provide a control to use for the growthprocess and appear to have excellent correlation with catalyst thicknessas well as the spinning capability of the resulting MWCNT forest. Usingthis kind of measurement can enable better comparison studies of therole of the catalyst film thickness on CNTs quality as well, since asmall difference in the Fe film thickness can induce large differencesin R_(s) as well as spinning capability.

By sectioning the wafer into chips, CNTs on a catalyst film have beengrown having a well defined R_(s) and as a consequence, a well definedcatalyst thickness. While the value of R_(s) on each chip wasapproximately constant, the range of R_(s) for different chips wasvaried from ˜9 MΩ/sq to ˜2 kΩ/sq. A set of conditions were utilized togrow the films so that the only variation during these experimentsoccurred in the catalyst film thickness. Those forest growth conditionswere flow rates of H₂/He/C₂H₂=100/20,000/700 sccm at a chip temperatureof 780° C. As shown in FIGS. 2A-2C, all of the MWCNT forests grown usingFe films having an R_(s) between 162 and 18.7 kΩ/sq were spin-capable.CNT sheets longer than 3 m could be readily pulled out from theseforests. (“Longer than 3 m” means either the sheets were stopped beingpulled after it passed 3 m length or that the sheet was pulled out ofthe forest until virtually all of the MWCNTs in the forest wereconsumed.)

The Fe film thickness corresponding to the R_(s) range of 162˜18.7 kΩ/sqwas 4 ˜6 nm by quartz crystal monitor. However the thickness was 5.0˜7.8nm when estimated from the fitting curve between R_(s) and the AFMthickness in FIG. 2C. This showed that the thickness range of theas-deposited Fe film was narrow (˜3 nm) for the growth of thespin-capable MWCNT forests.

FIG. 12A shows a sheet 1201 pulled from a multi-walled carbon nanotubeforest 1202 (such as shown in FIG. 11A). The sheet is over 3 m long.FIG. 12B shows a yarn 1203 pulled from a multi-walled carbon nanotubeforest. The yarn has a diameter of 300 nm.

The height of the CNT forests decreased from ˜420 μm to ˜275 μm as theR_(s) of the Fe film decreased from 150 kΩ/sq to 2.4 kΩ/sq. Given the 5minute growth period, these forest heights correspond to −1.4 μm/sec to−0.9 μm/sec growth rates, respectively. (FIG. 3H). It has been suggestedin previous reports [Wang 2007; Zhao B, Futaba D N, Yasuda S, AkoshimaM, Yamada T, Hata, K. Exploring advantages of diverse carbon nanotubeforests with tailored structures synthesized by supergrowth fromengineered catalysts. Acs Nano 2009 3:108-114] that the growth ratedecreases with increasing catalyst nanoparticle size, and this trend isalso observed in our results provided the nanoparticle size increaseswith increasing thickness of the as-deposited Fe film as expected.

The height of the forests grown on the chip with the largest R_(s) (9.6MΩ/sq) is a little less than that for 150 kΩ/sq. As shown in FIGS.3A-3G, the CNTs grown on the 9.6 MΩ/sq Fe film are more curled thanthose on the 150 kΩ/sq Fe film. The forest grown on the 9.6 MΩ/sq Fefilm has a CNT areal density of ˜1.4×10¹⁰, while that on the 150 kΩ/sqFe film has the areal density of ˜2.8×10¹⁰. When the CNT areal densityis low, the CNT forests are generally curled or wavy because neighboringCNTs are not close enough to have strong Van der Waals interactionsbetween CNTs. As a result, the overall height of the curled CNT forestgrown on the 9.6 MΩ/sq Fe film can be less than that of the well alignedforest grown on the 150 kΩ/sq Fe film, even if the CNT length of theformer can be longer than that of latter.

FIGS. 3A-3G has scanning electron microscope (SEM) images of the MWCNTsin seven forests grown on catalyst films of various R_(s). FIGS. 3C-3Eshow the CNTs forest morphology for spin-capable forests while FIGS.3A-3B and FIGS. 3F-3G show the CNTs forest morphology for those that cannot produce sheets or yarns. The spin-capable forests have CNTs that arewell aligned or even “super-aligned” parallel to one another. The othershave CNTs that appear more curled or wavy. In particular, the CNTs grownon chips with the largest R_(s) of 9.6 MΩ/sq or the lowest R_(s) of 2.4kΩ/sq appear the most curled. The ability to pull out CNTs is thought toresult from forces related to the alignment of the CNTs. This alignmentis strongly dependent on sheet resistance (and the thickness) of theas-deposited Fe film.

To examine the dependence of the average CNT diameter on the sheetresistance of as-deposited Fe film, the CNT diameter and itsdistribution were investigated by transmission electron microscopy(TEM). Regardless of the sheet resistance of the Fe films, all of theCNTs were MWCNTs (see FIGS. 9A-9G), which is consistent with the Ramanspectra of the CNTs (not shown here). Histograms of the averagediameters and standard deviation with Gaussian fitting are shown inFIGS. 9A-9G and summarized in Table 1. The tube diameter increasesslightly with decreasing R_(s). The average diameter of the CNTs isslightly smaller than the size of the Fe nanoparticles for each filmthickness. (This will be discussed further below.) It was also observedthat there are between 5 and 15 walls for each of the CNTs. This did notshow any significant dependence on R_(s).

The areal density of CNTs on the chips were compared in order to morefully understand how it is related to: the spinning capability of theforests, the alignment of the CNTs in the forest and the sheetresistance of the as-deposited Fe films.

TABLE 1 Average CNT diameters (and standard deviation) and distancebetween CNTs. Fe Films CNT diameter Areal density^(a) Area per CNTDistance between CNTs^(b) 9.6 MΩ/sq 11.0 ± 2.5 nm ~1.4 × 10¹⁰ ~85 nm ×85 nm ~74 nm 1.1 MΩ/sq 12.4 ± 2.9 nm ~1.9 × 10¹⁰ ~73 nm × 73 nm ~60 nm150 kΩ/sq 12.5 ± 1.3 nm ~2.8 × 10¹⁰ ~60 nm × 60 nm ~47 nm 70.0 kΩ/sq13.0 ± 2.4 nm ~2.3 × 10¹⁰ ~66 nm × 66 nm ~53 nm 18.8 kΩ/sq 12.7 ± 1.6 nm~1.7 × 10¹⁰ ~77 nm × 77 nm ~64 nm 11.0 kΩ/sq 13.1 ± 2.0 nm ~1.2 × 10¹⁰~91 nm × 91 nm ~78 nm 2.4 kΩ/sq 14.0 ± 1.9 nm ~8.7 × 10⁹  ~107 nm × 107nm ~93 nm ^(a)The average CNT areal density [tubes/cm²]. ^(b)The averagedistance between CNTs means the average distance between the outer wallsin MWCNT.

Spin-capable forests resulting from catalyst films with R_(s) between150 and 18.8 kΩ/sq had high areal density of CNTs. Table 1 showed thatthe density was greater than ˜2×10¹⁰ tubes/cm². This indicates that thelarge areal density promoted alignment of the CNTs, perhaps through Vander Waals interactions between growing CNTs. The areal density increasedfrom ˜1.4×10¹⁰ to ˜2.8×10¹⁰ tubes/cm² as R_(s) decreased from 9.6 MΩ/sqto 150 kΩ/sq. It peaked at 150 kΩ/sq, and then decreased again from˜2.8×10¹⁰ to ˜8.7×10⁹ tubes/cm² as R_(s) decreased from 150 kΩ/sq to 2.4kΩ/sq. The trend in the areal density may indicate two competingfactors. When R_(s) is small, the relatively thick Fe film formed verybig Fe nanoparticles that are not suitable for MWCNTs growth. When R_(s)was large, (relatively thin Fe films) the areal density decreasesbecause thin Fe films did not generate enough Fe nanoparticles suitablefor MWCNTs growth. Many of the nanoparticles produced were too small togrow MWCNTs.

The average distance between CNTs can be estimated using the CNTdiameter and the areal density. This average distance was about 47˜64 nmfor spin-capable forests (R_(s) between 150˜18.8 kΩ/sq) and was shorterthan for non-spin-capable forests. This combination of high arealdensity and large diameter CNTs promotes spin-capable forests andimplies that such forests have stronger Van der Waals interactionsbetween CNTs.

To confirm the root cause of these results Fe particle size wasexamined. This was done by annealing Fe films having R_(s) of 9.2 MΩ/sq,1.1 MΩ/sq, 150, 70.8, 17.8, 11.8 and 2.6 kΩ/sq. The anneal step wasperformed without the introduction of C₂H₂. FIGS. 4A-4G has AFM (*−1)and SEM (*−2) images showing the surface morphology of the annealed Fefilms with sheet resistance. Histograms of the Fe nanoparticle sizedistributions as a function of R_(s) are also shown in FIGS. 4A-4G andsummarized in Table 2.

The Fe nanoparticle size measured by AFM gradually increased from 14.3nm to 21.9 nm as the R_(s) of the Fe film decreased from 9.2 MΩ/sq to2.6 kΩ/sq. The morphology of the annealed Fe films was also measured bySEM because there are restrictions to the lateral extent of the AFMmeasurements and as a consequence some variations existed betweenmeasurement areas. Individual Fe particles were observed as shown inFIGS. 4A-4G, but the SEM could not distinguish nanoparticles withdiameters below ˜2 nm.

TABLE 2 Average Fe particle diameter (and standard deviation) ofannealed Fe film.^(a) Fe Films Particle diameter (AFM) Particle diameter(SEM) 9.2 MΩ/sq 14.3 ± 3.4 nm  4.9 ± 1.4 nm 1.1 MΩ/sq 15.3 ± 3.1 nm 11.4± 3.9 nm 150 kΩ/sq 17.5 ± 3.6 nm 13.3 ± 4.3 nm 70.8 kΩ/sq 19.1 ± 4.8 nm13.3 ± 4.3 nm 17.8 kΩ/sq 20.0 ± 4.1 nm 13.9 ± 4.1 nm 11.8 kΩ/sq 20.5 ±3.2 nm 16.4 ± 5.8 nm 2.6 kΩ/sq 21.9 ± 5.9 nm 21.9 ± 8.5 nm ^(a)Dataobtained on ~500 × 500 nm² in AFM or on ~500 × 700 nm² in SEM.

Even so, considering the CNT diameters observed in TEM and the minimumparticle size necessary for the growth of MWCNTs, one can consider thatnanoparticles below ˜2 nm in diameter can be neglected. The nanoparticlesize measured by SEM is similar to that by AFM with the lone exceptionoccurring for the 9.2 MΩ/sq film. Both measurement techniques indicatedthat the Fe particle size increased as the sheet resistance of Fe filmdecreased. In addition, the nanoparticle size measured using the SEMapproximately corresponded to the diameters of CNTs measured using theTEM. In addition, the Fe particles forming on the 9.2 kΩ/sq film weresmall and sparse like isolated islands in SEM images. This is consistentwith a forest that did not have sufficient nucleation sites for thegrowth of MWCNTs, and led to the low areal density as well as the curledCNT morphology.

The Fe nanoparticles in the 2.6 kΩ/sq film appeared relatively large,which is also consistent with a forest having fewer nucleation sites forthe growth of MWCNTs as mentioned above. This agrees with the previousresult that the presence of a large number of big particles decreasedthe density of carbon nanotubes resulting in worse alignment [Liu H,Cheng G, Zhao Y, Zheng R, Liang C, Zhao F, et al. Controlled growth ofFe catalyst film for synthesis of vertically aligned carbon nanotubes byglancing angle deposition. Surf Coat Technol 2006 201, 938-942]. As aresult, both the proper particle size, around 15 nm in SEM image, and asufficient density of Fe nanoparticles are required to have the highareal density of MWCNTs required for super-aligned and consequentlyspin-capable CNT arrays. Applicant believes that certain factors ofnanoparticle formation have deep impacts on the nature of the Fecatalyst film because the spinning capability of the MWCNT forestsstrongly depends on the size distribution and the density of the Fecatalyst.

Besides Fe film thickness, other keys to achieving sufficient arealdensity and optimally sized Fe nanoparticles are believed to exist. Ingeneral, H₂ gas has been used to reduce Fe oxide into Fe metal tofunction as an effective catalyst for the growth of CNTs. The differenceof the process with and without H₂ gas using Fe particle density andCNTs morphology as endpoints was tested to study the role of H₂ in theformation of Fe particles. CNTs were first grown under He gas withoutintroduction of H₂ gas on chips with R_(s) of 9.0 MΩ/sq, 150, 60.8,18.1, and 1.8 kΩ/sq. As shown in FIGS. 3A-3H and FIGS. 5A-5F, it wasfound that the CNTs in the resulting forests are more curled and shorterthan forests grown using a H₂/He gas mixture.

To investigate this phenomenon, chips having R_(s) of 9.2 MΩ/sq, 62.6,and 2.7 kΩ/sq were annealed using the same CNTs growth conditionswithout the introduction of C₂H₂ and H₂. As shown in FIGS. 6A-6C, the Fefilms annealed under He gas did not readily form nanoparticles, butinstead form plate-like films in the SEM images.

Such more-or-less contiguous films were not expected to be able to formthe proper nucleation sites for the growth of MWCNTs or have asufficient density of Fe nanoparticles unless they could be broken up insubsequent processing steps. This is consistent with the growth resultsshown above. These plate-like films apparently form when the Fe film wasannealed in an inert gas ambient (He). Since the addition of H₂ to theanneal gas promoted the formation of Fe nanoparticles, this suggeststhat iron oxide in or on the as-deposited Fe film must be active inpreventing Fe nanoparticle formation during the thermal anneal.

It is believed, then, that Fe_(x)O_(y) must help to maintain thecontinuity of the Fe film or at least to prevent the isolation ofindividual nanoparticles. Thus, reduction of iron oxide appears to havea important impact on the production of Fe nanoparticles. On the otherhand, hydrogen helped prevent the agglomeration of as-formed Fenanoparticles such as Ostwald ripening, which helps keep them small.Both models revealed that H₂ gas likely played a key role in generatingthe appropriate dimension Fe nanoparticles. This is significant to thegrowth of dense as well as controlled diameter MWCNTs. Given that CNTsgrowth did occur without H₂ in the anneal step, it appeared that C₂H₂can also reduce iron oxide to some degree and assist in the formation ofnanoparticles just before the initial nucleation step of the CNTs growth[Nishimura K, Okazaki N, Pan L, Nakayama Y. In situ study of ironcatalysts for carbon nanotube growth using x-ray diffraction analysis.Jpn J Appl Phys 2004 43:L471-L474].

It is believed that C₂H₂ gas before the initial nucleation step did notgenerate Fe nanoparticles as well as H₂ gas. This was why H₂ gas can begenerated from acetylene decomposed at Fe catalyst surface and thegenerated H₂ gas can reduce iron oxide [Perez-Cabero M, Taboada J B,Guerrero-Ruiz A, Overweg A R, Rodriguez-Ramos I. The role of alpha-ironand cementite phases in the growing mechanism of carbon nanotubes: a⁵⁷Fe Mossbauer spectroscopy study. Phys Chem Chem Phys 20068:1230-1235]. Such phenomenon can be investigated utilizing amechanistic study.

Electrical and Optical Properties of CNT Sheets

The electrical and optical properties of CNT sheets obtained from Fefilms were compared with R_(s) of 160, 62.0 and 19.0 kΩ/sq in order toinvestigate their feasibility for applications such as transparentelectrodes, flexible displays, and composites. The sheets tested werebetween 4 and 10 mm wide and between 8 and 14 mm long. Ethanol dropswere added onto the CNT sheets on top of a glass substrate after thesheets were pulled to make them adhere tightly. Subsequently, fourcopper tape electrodes were attached in order to perform a 4-point probemeasurement (see FIGS. 10A-10B). FIGS. 7A-7B show the outcome of theseexperiments.

The transmittance of the CNT sheet was plotted versus the sheetresistance of the same CNT sheet in FIG. 7A. It can be seen that thesheet resistance of a single CNT sheet did not vary much. The resistanceof a single sheet did not show a definite dependence on the R_(s) of theFe film (the different color data-markers) but all seemed to rangebetween 1.5˜3.0 kΩ/sq. Such phenomenon was not predicted. The CNTdiameter in the sheets was in the range of 12.5˜13.0 nm, and the forestheights were all in the range of 420 ˜375 nm. These were smalldifferences even though the R_(s) of the Fe film can vary by much more.As a consequence, the variation in the sheet resistances couldreasonably be small too. In addition, the variations which occur betweendifferent sheet samples from the same forest were significant.Therefore, any correlation between the R_(s) of the sheets and the R_(s)of the Fe film would be difficult to see. The optical transmittance at550 nm wavelength was in the range 0.78 to 0.89.

This transmittance is larger by ˜15%, and the sheet resistance is fourtimes larger than what has been reported in the literature [Liu 2008; MZhang 2005]. The difference most likely resulted from a difference inthe density of pulled CNTs in each sheet rather than any difference inthe CNT characteristics.

If instead of using a single sheet, (stacks) multiple sheets areoverlaid, then the resistance and transmittance of the multi-sheetdecreased. Multi-sheets were prepared from MWCNT forests grown from anR_(s)=160 kΩ/sq Fe film by overlaying either 3, 5 or 10 single sheets.The resulting resistance and transmittance (at 550 nm) of themulti-sheets are shown in FIG. 7B. It can be seen that both thetransmittance and resistance decreased as the number of overlaid sheetsincreased, but in a nonlinear fashion. In addition, the multi-sheetswere found to be more consistent in both resistance and transmittance.The large variation in the resistance of a single sheet averaged out ina multi-sheet quickly.

FIGS. 16A-16D are MWCNT sheets (1, 3, 5, and 10, respectively)transferred onto glass substrates with the corresponding I-V curvemeasured by 4-point probe. FIG. 16E shows the optical transmittance as afunction of wavelength of these MWCNT sheet of FIGS. 16A-16D. FIG. 16Fshows the optical transmittance at 550 nm of MWCNT sheets as a functionof the electrical sheet resistance of these MWCNT sheet of FIGS.16A-16D.

The SEM images in FIGS. 8A-8D indicated that each single sheet consistsof aligned CNT bundles so that the MWCNT density had significantvariation across the sheet.

This locally dense structure led to the large variation in the sheetresistance and transmittance of each sheet. When sheets were overlaid,the resulting multi-sheet density became larger and more uniform,leading to both a lower sheet resistance and a lower transmittance withsmaller variations.

The relationship between the spinning capability of MWCNT forests andthe behavior of Fe catalyst films have been characterized by measuringthe sheet resistance of the Fe catalyst films and correlating it to AFMthickness measurements. The spinning capability depends on the alignmentof adjacent MWCNTs in the forest which in turn results from thesynergistic combination of a high areal density of MWCNTs and shortdistance between the MWCNTs. This can be realized by starting with boththe proper nanoparticle size at a sufficient density. In particular, thestarting thickness of an Fe film along with H₂ gas in the ambient duringthe anneal process both play key roles in controlling the nanoparticledensity. Proper Fe catalyst film thickness and the use of H₂ to reduceany iron oxides are significant factors for generating suitably sized Fenanoparticles. The optical transmittance and sheet resistance of singleMWCNT sheets showed significant variations between samples. Whenmultiple sheets were laid on top of one another, the multi-sheet, sheetresistance and optical transmittance were both reduced and much moreconsistent. This is opens a practical route for controllable andreproducible spin-capable MWCNT forests.

Mechanism MWCNTs Are Pulled From Forests

FIG. 13 illustrates the believed mechanism in which MWCNTs are pulledout from the multi-walled carbon nanotube forests. This mechanisminvolves Steps I-V. FIGS. 14A-14J and FIGS. 15A-15B are TEM images thatmodel the pulling the sheet from a multi-walled carbon nanotube forest.

FIG. 14A shows a bundle of carbon nanotubes 1400 pulled out from a MWCNTforest. FIG. 14B shows a magnified view of the carbon nanotubes in block1401 of FIG. 14A. FIG. 14C shows a magnified view of the carbonnanotubes in block 1403 of FIG. 14B. As seen in block 1403, the bundlesare tangled at the top of the nanotube forest (Step I to Step II asshown in FIG. 13). FIG. 14D shows a magnified view of the carbonnanotubes in block 1405 of FIG. 14C.

FIG. 14E shows a magnified view of the carbon nanotubes in block 1404 ofFIG. 14B. As shown in block 1404, bundles are peeled off the MWCNTforests downwards (Step II to III as shown in FIG. 13).

FIG. 14F shows a magnified view of the carbon nanotubes in block 1402 ofFIG. 14A. FIG. 14G shows a magnified view of the carbon nanotubes inblock 1407 of FIG. 14F. As shown in block 1407, bundles are stuck to thebottom of the MWCNT forests (Step III as shown in FIG. 13).

FIG. 14H shows a magnified view of the carbon nanotubes in block 1408 ofFIG. 14G. As shown in block 1408, bundles are tangled at the bottom ofthe MWCNT forests (Step IV as shown in FIG. 13).

FIG. 14I shows a magnified view of the carbon nanotubes in block 1409 ofFIG. 14H. As shown in block 1409, MWCNT tips pulled out from the bottomend of the CNT forests. FIG. 14J shows a magnified view of the carbonnanotubes in block 1406 of FIG. 14F. As shown in block 1406, bundles repeeled off the MWCNT forest upwards (Step IV to V as shown in FIG. 13).

FIG. 15A shows a sheet 1501 pulled out from a MWCNT forest. FIG. 15Bshows a magnified view of the carbon nanotubes in block 1502 of FIG.15A. As shown in block 1502, the tangled parts in the MWCNT forest arepreserved even after being pulled out in the form of sheets (Step V asshown in FIG. 13).

Applications

These potential applications of the MWCNTs yarns and sheets arewidespread and include, for example, flexible electrodes (such as shownin FIG. 17), nanowires (such as shown in FIG. 18), photovoltaic cells(such as shown in FIGS. 19-20), and solar cells (such as shown in FIGS.22-24).

For instance, referring to FIG. 19, a photovoltaic cell can include ahole-conducting layer 1908 that has a hole-collecting material 1902,MWCNT sheets or yarns 1903, and hole collecting material 1904.Additionally, the photovoltaic cell can include a hole-conductingelectrode 1901, a photovoltaic layer 1905, an electron-collectingelectrode 1906, and a substrate 1907.

In an alternative embodiment (illustrated in FIG. 20), the photovoltaiccell is arranged similarly except for the absence of hole-collectingmaterial 1902 in the hold conducting layer 1908.

FIGS. 21A-21C are illustrations of MWCNT sheet that can be utilized as ahole conducting layer in a photovoltaic cell. FIG. 21A is a top view ofa one-dimensional structure. FIG. 21B is a top view of a two-dimensionalstructure. FIG. 21C is a side view of the two-dimensional structure ofFIG. 21B.

For instance, referring to FIG. 22, a solar cell can include ahole-conducting electrode 2201, a hole-collecting material 2202, a metaloxide/CNT sheet composite 2203, hole collecting material 2204, aphotovoltaic layer 2205, an electron-collecting electrode 2206, and asubstrate 2207.

In an alternative embodiment (illustrated in FIG. 23), the solar cell isarranged similarly except for the absence of hole-collecting material2202.

In an alternative embodiment (illustrated in FIG. 23), the solar cell isarranged similarly except for the absence of hole-collecting material2202 and hole collecting material 2204 and the addition of anotherphotovoltaic layer 2401.

FIG. 25 illustrates a metal oxide/CNT sheet composite 2501 that can beutilized in the solar cells illustrated in FIGS. 22-24. Metal oxide/CNTsheet composite 2501 includes metal oxide 2502 and CNT sheet 2503. Forexample, such metal oxide/CNT sheet composites can be a manganeseoxide/CNT sheet composite, a cobalt oxide/CNT sheet composite, and anickel metal/CNT sheet composite, such as shown in FIGS. 26A-26C,respectively.

The examples provided herein are to more fully illustrate some of theembodiments of the present invention. It should be appreciated by thoseof skill in the art that the techniques disclosed in the examples whichfollow represent techniques discovered by the inventors to function wellin the practice of the invention, and thus can be considered toconstitute exemplary modes for its practice. However, those of skill inthe art should, in light of the present disclosure, appreciate that manychanges can be made in the specific embodiments that are disclosed andstill obtain a like or similar result without departing from the spiritand scope of the invention.

All patents and publications referenced herein are hereby incorporatedby reference. It will be understood that certain of the above-describedstructures, functions, and operations of the above-described embodimentsare not necessary to practice the present invention and are included inthe description simply for completeness of an exemplary embodiment orembodiments. In addition, it will be understood that specificstructures, functions, and operations set forth in the above-describedreferenced patents and publications can be practiced in conjunction withthe present invention, but they are not essential to its practice. It istherefore to be understood that the invention may be practiced otherwisethan as specifically described without actually departing from thespirit and scope of the present invention.

1. A method comprising the steps of: (a) depositing a catalyst film on asubstrate to form a catalyst-incorporated substrate, wherein thecatalyst film has a sheet resistance in the range of about 10 kΩ/squareto about 1 MΩ/square; (b) measuring the conductivity, sheet resistance,and resistivity of the catalyst film; (c) growing a CNT forest on thecatalyst-incorporated substrate; (d) pulling out a bundle of CNTs fromsaid CNT forest; and (e) forming a CNT sheet or yarn.
 2. The method ofclaim 1, wherein the step of measuring comprises using a four-pointprobe.
 3. The method of claim 1, wherein the step of measuring comprisesusing an eddy current probe.
 4. The method of claim 1, wherein thecatalyst film comprises iron and wherein the sheet resistance of thecatalyst film is in the range of about 10 kΩ/square to about 1MΩ/square.
 5. The method of claim 1, wherein the substrate comprises anoxidized material selected from the group comprising silicon, metals,metal oxides, glass, and combinations thereof.
 6. The method of claim 1,wherein the catalyst film comprises an iron deposited on the substrateby a process selected from the group consisting of electron-beamevaporation, sputtering, and chemical vapor deposition.
 7. The method ofclaim 1, wherein the bundle of CNTs comprise multi-walled CNTs.
 8. Themethod of claim 1, wherein the step of growing the CNT forest comprises:(i) putting the catalyst-incorporated substrate on a heater block in achemical vapor deposition (CVD) chamber; (ii) heating the CVD chamber upto a temperature for the growth of CNT forest; (iii) holding thesubstrate in the CVD chamber at the temperature for the growth of CNTforest; (iv) providing a gas mixture consisting of CNT-formingprecursors and carrier gases; and (v) growing the CNT forest on thesubstrate such that the CNT forest are substantially aligned in adirection perpendicular to the substrate.
 9. The method of claim 8,wherein the hold temperature of the heater block in the CVD chamber isbetween room temperature and the temperature for the growth of CNTforest.
 10. The method of claim 8, wherein the temperature for thegrowth of the CNT forest is between about 650° C. and about 850° C. 11.The method of claim 8, wherein the heating rate up to the temperaturefor the growth of CNT forest is between about 10° C./minute and about1000° C./minute.
 12. The method of claim 8, wherein the substrate thatis heated up to the temperature for the growth of CNT forest stays lessthan 30 minutes in carrier gases.
 13. The method of claim 8, whereinbefore growing the CNT forest, carrier gases is supplied into the CVDchamber.
 14. The method of claim 8, wherein the CNT-forming precursorcomprises acetylene, and the carrier gases comprise hydrogen and aninert gas.
 15. The method of claim 14, wherein the inert gas is selectedfrom the group consisting of helium, nitrogen, and argon.
 16. The methodof claim 8, wherein (i) the flow rate of the acetylene is between about200 sccm and about 1000 sccm; (ii) the flow rate of the hydrogen isbetween about 50 sccm and about 1000 sccm; and (iii) the flow rate ofthe inert gas is between about 1 slm and 30 slm.
 17. The method of claim1, wherein (i) CNTs of the bundle of CNTs comprise multi-walled CNTsthat are entangled end to end, and (ii) the multi-walled CNTs aresubstantially parallel to each other along the drawing direction.
 18. Amethod of forming a hole conducting layer in a photovoltaic cellcomprising the steps of: (a) forming a first electrode; (b) forming anelectron donor/acceptor composite layer; (c) forming a hole conductinglayer stacked with a hole conducting material over CNT sheets or yarns;and (d) forming a second electrode on the contrary.
 19. The method ofclaim 18, wherein the hole conducting material comprisespoly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), indium tinoxide (ITO) and p-type transparent metal oxides.
 20. The method of claim19, wherein the p-type transparent metal oxide has a work function ofover 4.5 eV.
 21. The method of claim 20, wherein the p-type transparentmetal oxide is selected from the group consisting of iron oxide, nickeloxide, tin oxide, vanadium oxide, zinc oxide, and combinations thereof.22. The method of claim 18, wherein the CNT bundles in the CNT sheet oryarn is substantially aligned in the same direction or cross each otherperpendicularly.
 23. The method of claim 18, wherein the hole conductinglayer is a triple-layer in which the CNT sheets or yarns is laid betweenthe hole conducting materials.
 24. A method of incorporating a holeconducting material into an electron donor/acceptor composite layer in aphotovoltaic cell comprising the steps of: (a) forming a firstdonor/acceptor composite; (b) forming a hole conducting layer thatcomprises CNT sheets, wherein the CNTs are encapsulated by metal oxidescomprised of metal oxide/CNT sheet composites; and (c) forming a seconddonor/acceptor composite.
 25. The method of claim 24, wherein the metaloxide is a p-type transparent metal oxide with work function of over 4.5eV.
 26. The method of claim 25, wherein the p-type transparent metaloxide is selected from the group consisting of iron oxide, nickel oxide,tin oxide, vanadium oxide and zinc oxide.
 27. The method of claim 24,wherein the metal oxide comprises a transition metal oxide.
 28. Themethod of claim 24, wherein the metal oxide is coated onto the CNTs by aprocess selected from the group consisting of sputtering,electro-deposition, chemical vapor deposition (CVD), and combinationsthereof.
 29. The method of claim 24, wherein (i) the CNTs areencapsulated by metal oxides by a process comprising coating a metalonto the CNTs, wherein the process is selected from the group consistingof sputtering, electro-deposition, CVD, and combinations thereof; and,(ii) oxidizing the metal using oxidants.
 30. The method of claim 29,wherein the oxidants are selected from the group consisting of oxygen,ozone, hydrogen peroxide, and nitric acid.
 31. A method of manufacturingan anode for fuel cells and lithium batteries comprising the step of:(i) pulling out a bundle of CNTs from said CNT forest; (ii) forming CNTsheets, (iii) forming metal oxide/CNT sheet composites comprising CNTsheets wherein the CNTs are encapsulated by metal oxides.
 32. The methodof claim 31, wherein the metal oxide comprises a transition metal oxide.33. The method of claim 32, wherein the transition metal oxide isselected from the group consisting of manganese oxide, nickel oxide,cobalt oxide, and iron oxide.
 34. The method of claim 31, wherein themetal oxide is coated onto the CNTs by a process selected from the groupconsisting of sputtering, electro-deposition, chemical vapor deposition(CVD), and combinations thereof.
 35. The method of claim 31, wherein (i)the CNTs are encapsulated by metal oxides by a process comprisingcoating a metal onto the CNTs, wherein the process is selected from thegroup consisting of sputtering, electro-deposition, CVD, andcombinations thereof; and, (ii) oxidizing the metal using oxidants. 36.The method of claim 35, wherein the oxidants are selected from the groupconsisting of oxygen, ozone, hydrogen peroxide, and nitric acid.